360 GEOLOGY 



Causes of Secular Movements 



The volume of the earth depends on two sets of forces, acting 

 in opposition to one another, (1) the concentrating forces, consisting 

 of (a) gravity and (b) molecular and sub-molecular attractions, 

 and (2) the forces which resist concentration, consisting of (a) heat 

 and (b) molecular and sub-molecular resistances. 



1. The centripetal forces. The best known of the concentrating 

 forces is gravity, which tends to bring all parts of the earth as near 

 the center as possible, the heavier beneath the lighter. 1 The 

 gravitative force of the earth causes a pressure of about 3,000,000 

 atmospheres at its center, and a graded series of lesser pressures 

 from the center to the surface. This is a constantly acting force 

 and tends to bring about greater density whenever any molecular 

 movement permits it. 



In addition to gravity, there are attractive agencies acting be- 

 tween molecules, atoms, ions, and electrons, which co-operate with 

 gravity in accordance with laws of their own. Their general effect 

 is to give matter denser forms. How far these may go by united 

 and prolonged action is not determined, but there is ground for 

 believing that the density of the interior may still be increased by 

 such action. It is known that substances which crystallize in a 

 given way under surface pressures may be re-aggregated into denser 

 forms under higher pressures. It is a general law that re-aggregation 

 under pressure takes the densest form available. Re-aggregation in 

 the interior thus probably means increased density, and it may be 

 constantly going on. It has even been suggested that complex 

 molecules may be organized under such conditions as obtain in the 

 earth's interior, but for the present this must be regarded as merely 

 speculative. 



While we must hold final judgment in suspension until fuller 

 knowledge of the laws of molecular, atomic, and sub-atomic com- 

 bination under extremely high pressures and temperatures is 

 developed, it is permissible even now to entertain the view that 

 gravitational, molecular, atomic, and sub-atomic agencies have 

 been and are still at work tending to increase internal density. 



1 For distinctions between gravitational energy and gravitational force 

 see the authors' larger work, Vol. II, pp. 552-553. 



